1. Field of the Invention
The present invention relates to color signal processing for color video signals and, in particular, to an image pickup system and image processing program for obtaining high quality video signals by correcting chroma signals in accordance with a tone conversion of luminance signals.
2. Description of Related Art
The main stream of current image pickup systems such as digital still cameras and video cameras uses a single plate CCD having a complementary color filter or a primary-color filter on the front side.
In this kind of image pickup system, white-balance (WB) processing and/or interpolation processing are performed on color signals from a CCD. Then, picture creating processing such as edge emphasizing processing, chroma emphasizing processing and tone correcting processing is performed thereon, and the result is output.
The resulting signals may further undergo chroma-suppress processing, highlight cyan processing and so on. The chroma-suppress processing is performed for reducing a false color, which may occur at the edge, in the interpolating processing. The highlight cyan processing is performed for reducing a false color, which may occur at a part having a high intensity due to a difference in spectral sensitivity characteristic of a CCD.
The picture-creating processing as described above may be directly performed on color signals from a CCD. However, in general, a color signal is converted into a different color space and is separated into a luminance signal and color signals to undergo the processing thereafter. In this case, the color signals may deviate from a gamut due to the chroma emphasizing processing, for example, or the color signals may become unnatural when only the luminance signal is tone-converted, which are problems and need some solutions.
In order to solve the problem of the Gamut deviation, Japanese Patent No. 2567214, for example, discloses processing for obtaining a chroma histogram for each hue and compressing the entire form of the histogram such that the histogram can fit in a gamut.
Also, Japanese Patent No. 3190050 discloses processing for compressing a chroma for each hue toward the origin point so as to fit in a gamut.
On the other hand, in order to solve the problem that color signals become unnatural, Japanese Unexamined Patent Application Publication No. 2001-238129, for example, discloses a technology for correcting color signals so as to provide a fixed ratio for a theoretical characteristic value of a gamut when a luminance signal is changed.
When the technology for compressing the entire form of a histogram as disclosed in Japanese Patent No. 2567214 and/or the technology for compressing a chroma toward the origin point as disclosed in Japanese Patent No. 3190050 are used, the image does not deviate from the gamut as a whole. However, the color reproduction after the processing depends on the image to be processed. Thus, the picture creating processing intended for chroma emphasis and so on is not reflected thereon, and the control of the color reproduction becomes difficult, which is another problem.
By using the technology for providing a fixed ratio for a theoretical characteristic value of a gamut as disclosed in Japanese Unexamined Patent Application Publication No. 2001-238129, natural color reproduction can be achieved. However, the amount of calculation is large, and as the length of the processing time increases, the cost of the entire apparatus increases, which is another problem.